Influenza A and B are the two types of influenza viruses that cause epidemic human disease (111). Influenza A viruses are further categorized into subtypes on the basis of two surface antigens: hemagglutinin (HA) and neuraminidase (NA). Influenza B viruses are not categorized into subtypes, but do under go drift whereby strains diverge over time. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have been in global circulation. Influenza A (H1N2) viruses that probably emerged after genetic reassortment between human A (H3N2) and A (H1N1) viruses have been detected recently in many countries. Both influenza A and B viruses are further separated into groups on the basis of antigenic characteristics. New influenza virus variants result from frequent antigenic change (i.e., antigenic drift) resulting from point mutations that occur during viral replication. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses. Frequent development of antigenic variants through antigenic drift is the virologic basis for seasonal epidemics and the reason for the incorporation of at least one new strains in each year's influenza vaccine.
A person's immunity to the surface antigens, especially hemagglutinin, reduces the likelihood of infection and severity of disease if infection occurs (112). It is generally thought that antibody against one influenza virus type or subtype confers limited or no protection against another. Furthermore, it is generally accepted that antibody to one antigenic variant of influenza virus might not protect against a new antigenic variant of the same type or subtype (113). Therefore, the demonstration of cross-protection is unexpected.
Human-avian reassortant influenza viruses were responsible for the previous two influenza pandemics in 1957 and 1968. Since H2 viruses have not circulated in humans after 1968, an antigenic shift arising from an H2 reassortant virus is theoretically possible at any time. However, the recent emergence of highly pathogenic avian influenza (HPAI) viruses (H5 and H7) and the sporadic transmission of these viruses directly from birds to humans since 1997 (1-5) brings a new human pandemic threat potential in addition to the population's ever increasing susceptibility to H2 viruses. The fact that human HPAI H5N1 outbreaks have been antigenically distinct makes it all but impossible to prepare advance stockpiles of a well-matched vaccine against a pandemic threat (5, 6). While mouse H5 immunization and challenge data indicate that good cross reactivity is seen between various H5 isolates in this model (7), it is not known if similar levels of cross reactivity will be seen in humans with existing vaccine technology. Thus, there is a need for influenza vaccine platforms that may be quickly adapted to include antigens from new viral outbreaks.
The present egg-based inactivated vaccine technology is inadequate to meet the demands of an emerging pandemic due to the inability to propagate HPAI viruses in eggs and the need for enhanced biocontainment (6, 8). Reverse genetics approaches offer a means of producing low pathogenicity reassortants with the desired HA and NA makeup that can be cultured in eggs (7, 9-11); however, vaccines produced by this approach are only now entering the clinic due to previous intellectual property and regulatory issues (8). An additional concern is the apparent low level immunogenicity associated with H5 hemagglutinins evaluated in human clinical trials (12-14) which makes it clear that improved vaccines, delivery systems, and the use of adjuvants may be required to efficiently induce protection in a population that is completely H5-naïve. Thus, there is a need for an influenza vaccine platform that allows for expression of HPAI antigens in combination with adjuvants.
Influenza VLPs represent an alternative technology for generating influenza vaccines. Influenza VLPs have been produced using the influenza matrix, HA and NA proteins expressed in insect cells which are markedly immunogenic following intranasal delivery (26, 27). Indeed, VLPs in general appear well suited for the induction of mucosal and systemic immunity following intranasal delivery as has been shown for rotavirus, norovirus, and papilloma virus VLPs (28-31). Influenza VLPs have been produced in eukaryotic expression systems by expression of influenza matrix, HA and NA proteins. The influenza matrix is the driving force behind virus budding and NA is required for budded VLP release from producer cells when HA is also being expressed owing to HA's association with sialic acid at the cell surface (51). There are also data to indicate that interactions between matrix and the C-terminus of HA play a role in directing matrix to the membrane as part of the budding process (51). Influenza VLPs produced in an insect cell baculovirus expression system have proven immunogenic in animal trials and represent an important strategy for future pandemic preparedness (26, 27, 47). In addition, intranasal delivery of influenza VLPs can result in antibody titers exceeding those obtained following parenteral administration. However, preliminary data indicated that use of the matrix protein in generating influenza VLPs resulted in poor yields of VLPs which renders the matrix derived influenza VLPs a poor choice to date for an alternate form of influenza vaccine. This is consistent with more recent data showing that influenza virus particle assembly is complex and that the matrix protein is really not the driving force behind particle assembly (Chen et al J Virol. 2007 July; 81 (13):7111-23). Thus, there is a need for an influenza vaccine platform that can generate sufficient quantities of VLPs for vaccine production. In addition, there is a need for an influenza vaccine platform that can provide drift and heterosubtypic protection against influenza to rapidly provide protection against new emerging strains and subtypes before better matched vaccines can be developed and produced.